The present invention relates to a rotary electric device which comprises a rotary mechanism portion including rotor means and stator means contained in a case, and more particularly, it relates to an improvement in a closed type rotary electric device. Examples of such a rotary electric device are a trimmer capacitor, a variable resistor, a variable coil and a rotary switch etc., and the present invention may be applied to any of these rotary electric devices.
A rotary electric device comprises a rotary mechanism portion including a rotor means and a stator means contained in a case. In such a rotary electric device, the rotor means is rotated to change, e.g., the capacity in a trimmer capacitor and the resistance value in a variable resistor. Therefore, the case of the rotary electric device is provided with an opening for performing desired adjustment by changing characteristics of the rotary electric device from the exterior thereof, wherefore the rotor means is rotated through the opening.
Generally when a rotary electric device is mounted on a printed circuit board etc., flux employed in connection with a soldering operation may enter the case through the opening formed therein to exert bad influences on the characteristics of the rotary electric device. Further, for improving the operation of the device and reducing mass production costs, it is desired to pass all of electric or electronic components through a molten solder tank to simultaneously solder a plurality of electric or electronic components on the printed circuit board, etc. In such a case, however, the solder inevitably enters the rotary electric device through the opening formed in the case to disadvantageously influence the rotor means and stator means etc., and as the result, for example, rotation of the rotor means might be prevented and characteristics of a dielectric member or resistance film serving as the stator means might be degraded. Thus, it has generally been necessary to solder the rotary electric device such as a trimmer capacitor onto the circuit board after the board has been passed through the molten solder tank.
To avoid this problem, there has been provided the so-called closed type rotary electric device to which a cover sheet is applied to close an opening defined in its case so as to prevent flux employed in soldering and solder in employment of a molten solder tank from entering the case of the rotary electric device. Such a closed type rotary electric device is disclosed in, for example, Japanese patent laying-open gazette No. 72353/1983 by the inventor et al., which is in the structure as shown in FIGS. 1 and 2.
FIGS. 1 and 2 show a prior art trimmer capacitor. FIG. 1 is a plan view and FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1.
A case 1 of the trimmer capacitor is formed with two terminals 2 and 3 embedded therein. The case 1 is preferably made of material resistant to solder melting temperatures, such as heat-resistant thermosetting resin which is resistant to a temperature of about 300.degree. C. An end of the terminal 2 located within the case 1 is in the form of a throttle grommet 2a, which provides a center shaft for rotatably supporting a rotor 4. An end of the other terminal 3 located within the case 1 is so provided as to be partially exposed from the bottom surface of the case 1. The terminals 2 and 3 are respectively extracted outwardly from the case 1.
The case 1 has an opening 5 and contains a rotary mechanism portion located within the opening 5. The rotary electric device as shown in FIGS. 1 and 2 is a trimmer capacitor, and the rotary mechanism portion is formed by a spring 6, a dielectric member 7 and a rotor 4 successively located from the bottom in FIG. 2. The spring 6 is so arranged as to be electrically connected with the exposed end of the terminal 3, and is ring-shaped in plan view. The dielectric member 7 positioned stationarily on the upper surface of the ring-shaped spring 6 is provided in its lower surface with a semicircular electrode conductor, which is in contact with the spring 6. The dielectric member 7 is contained in the case 1 in a state which prevents it from rotating. The rotor 4, which is in contact with the upper surface of the dielectric member 7, is integrally provided in its lower surface with a semicircular rotor electrode 4a. The throttle grommet 2a passes through the central portion of the rotor 4 to rotatably retain the rotor 4 while the upper end thereof is caulked to retain the spring 6, the dielectric member 7 and the rotor 4 within the case 1. The rotor 4 is provided in its upper surface with an adjusting groove 4b, into which a tool such as a screwdriver may be placed rotate the rotor 4. Upon such rotation of the rotor 4, the area of opposition between the rotor electrode 4a and the electrode conductor provided in the lower surface of the dielectric member 7 is changed to vary the capacitance of the device.
A cover sheet 8 made of, e.g., polyimide resin is attached to an end surface 1a of the case 1 in which the opening 5 is defined. The cover sheet 8 is illustrated as being transparent in FIG. 1 to permit viewing of the interior of the case 1. It is to be noted that the cover sheet 8 may as a practical matter be transparent. The cover sheet 8 is made of heat-resistant material which is resistant to, e.g., a temperature of about 300.degree. C. Further, the cover sheet 8 is preferably formed by material having such thickness and strength that it can be easily broken by a tool such as a screwdriver, so that the capacitance of the device is readily controlled using breaking the cover sheet 8 by the screwdriver, etc. The cover sheet 8 is attached to the end surface 1a of the case 1 by a heat-resistant adhesive such as an epoxy adhesive. The cover sheet 8 may be attached to the end surface 1a of the case 1 by various means such as adhesion.
While the rotary electric device described above is somewhat satisfactory, it exhibits the following problems. When the rotor 4 is rotated, the cover sheet 8 is irregularly broken whereby fragments of the broken cover sheet 8 may be scattered. Such scattering of the cover sheet fragments causes dust to enter the rotary electric device upon breaking of the cover sheet 8. Further, the fragments of the cover sheet 8 may cause significant circuit fault when they are scattered on other circuits arranged in the vicinity of the rotary electric device.